Which of the following is NOT a valid guideline for creating indexes?

a$.$ Declare primary and foreign keys so the optimizer can use the indexes in join operations.

b$.$ Create indexes for each single attribute used in a WHERE, HAVING, ORDER BY$,$ or GROUP BY clause.

c$.$ Declare indexes in join columns other than primary or foreign keys.

d$.$ Do not use indexes in large tables or tables with high sparsity.